Television receiver video circuit



Oct. 7, 1941.

OQRREER FREQUENCY M. K. TAYLOR ET AL 2,258,370

TELEVISION RECEIVER VIDEO CIRCUIT Filed May 51, 1939 37 FL 230' zoj 1 n Y (b LINE OFREouENcY GENERATOI?) I I33 I v O O gINOHRONIZIIg FIELD SIGNAL -OFREQUENOY "'35 SEPARATOR GENERATORO i A c I J INVENTOR MAURICE KENYON TAYLOR BY HERBERT WOOD ATTORNEY Patented Oct. 7, 1941 2,258,370 TELEVISION RECEIVER. vnmo cmcmr Maurice Kenyon Taylor and Hubert Wood, Hollinwood, England, assignors to Hazeltine Corporation, a corporation of Delaware Application May 31,1939, Serial No. 276,736 In Great Britain May 30, 1938 6 Claims. (01; 178-75) (Granted under the provisions of sec. 14, act of March 2, 1927: 357 0. G. 5)

This invention relates generally to television receivers and more specifically to such receivers 01 the type employing a cathode-ray tube for the reproduction of a scene transmitted in accordance with a system in which light variations of the scene are represented by variations in carrier-wave amplitude. a

It is usual intelevision receivers of the abovementioned type to-apply the output of a videofrequency amplifier between the control electrode and the cathode of a cathode-ray tube in order to modulate the electron" stream, the oathode of the tube being held at a fixed potential. When directly heated tubes are used to rectify the high-voltage supply ofthe receiver, such receivers have the disadvantage that there is generally lnsufflcient bias for the control electrode of the cathode-ray tube during the warming up means for maintaining the cathode ray extinguished during the' aforesaid warm-up period comprising means responsive to the space current of the translating stage vacuum tube for period of the receiver tubes. During this period the cathode of the cathode-ray tube of such a system tends to supply an excessivev emission current and may, therefore, be'damaged. Further, until the "time-bases or scanning fields of the receiver. commence to operate, which operation is delayed due to the initial warm-up or cathode V heating period of the thermionic tube means employed therein, a bright spot is formed on .the fluorescent screen of the tube and the screen may be burned in the region of the spot and permanently damaged. A further disadvantage of the above arrangement is that degeneration of the cathode-ray tube used for image reproduction may be introduced by reason of the resistance generally included in series with its cathode and the negative terminal of'the hig voltage supply of the tube.

It is an object of the present invention to provide a television receiver of the type described which does not suffer from the disadvantages set forth above. I

-In accordance with the invention, therefore, a television receiver of the type described combiasing the cathode-ray tube to cutoff for values of space current below a predetermined mini-' mum value and for reducing the bias of the cathode-ray tube to a normal operating value for values of the space current in excess of the aforesaid predetermined minimum value, the space current of the translating stage vacuum tube being less than the aforesaid predetermined minimum value during its warm-up period and the translating stage vacuum tube having a warmup period not less than the warm-up period of the scanning tube means.

For a better understanding of the invention, 7

together with. other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

The single figure of the drawing is a circuit diagram, partly schematic, of a complete television receiver in accordance with the invention.

Referring now more particularly to the drawing, the system illustrated comprises a complete television receiver including an antenna 3|, 32 to which is connected in cascade, in .the order named, a carrier-frequency translator 30, a detector E, a video-frequency signal-translating stage, such as a thermionic vacuum tube amplifier 8, and an image-reproducing device such as a cathode-ray tube Ill. The receiver includes means for producing scanning fields to deflect the cathode ray of the cathode-ray tube 18 in accordance with a predetermined scanning'pattern, this means comprising a line-frequency prises a. signal-translating stage including a thermionic vacuum tube, a cathode-ray tube for sufficient scanning fields'to deflect the cathode ray of the cathode-ray tube, thereby tending to damage the latter. The receiver also-includes generator 34v and a field-frequency generator'35 which are coupled to the output circuit of de- 'tector 4 through a synchronizing-signal-separating device 33, the line-frequency generator .34

and the field-frequency generator 35 being coupled to the line-deflecting coils 36 and'the fielddeflecting coils 31- of the receiver,.respectively. It will be understood that units 34 and '35 in- 'clude in conventional manner thermionic vacuum-tube means having an initial warm-up period during which there are. produced insuflicient scanning fields to deflect the cathode-ray beam of tube I 8, thereby tend-ing to damage the latter.

The stages or units 30-31, inclusive, may all be of well-known construction so that a detailed illustration and description thereof are deemed unnecessary herein. The translator 30 may comprise carrier-frequency conventional radio-frequency stages, an oscillator-modulator nals intercepted by antenna circuit 3i, 32 are selected, amplified, and delivered as a carrierfrequency signal to detector 4. The modulation components of the signal are derived by detector 4 and are supplied to video-frequency amplifier 8, wherein they are amplified and from which they are supplied to the brilliancy-control circuit of image-reproducing device l8. Synchronizing signals derived from the load circuit of detector 4 are separated by synchronizing-signal separator 33 and are supplied to the synchronizing control elements of generators 34 and 35. The intensity of the scanning ray of cathoderay reproducing device i8 is thus modulated or controlled in accordance with the video-frequency voltages impressed upon its input circuit in the usual manner. The scanning waves generated in the line-frequency and field-frequency generators 34 and 35, which are controlled by synchronizing voltage pulses supplied from synchronizing-signal separator 33, are applied to the scanning elements of image-reproducing device I8 to produce electric scanning fields, thereby to deflect the scanning ray in two directions normal to each other soas to trace a rectilinear pattern on the screen and thereby reconstruct the transmitted image,

' 3 of detector 6 by means of a resistor 9 coupled to a tap on a resistor it) which is connected across a filter reactor ll, one end of which is connected to the negative terminal N of a direct current source PN. The anode I 3 of video-frequency amplifier 8 is connected to the positive terminal P of the direct current source PN through a load circuit comprising a seriesconnected inductor I4 and resistor [5, while its indirectly-heated cathode I6 is grounded. Cathode-ray tube I 8 comprises an electron gun, which may be of conventional construction, for developing, focusing, and accelerating a cathode-ray beam and comprising at least an indirectlyheated cathode IT, a control grid 20, and an anode 23. In order to maintain the control 'grid 20 of cathode-ray tube l8 at a substantially uni-'- form potential during reproduction of a received signal, it is connected by way of grid-leak resistor 2| to a tap on a resistance potential divider 22 connected across source PN through the filter reactor II. The potential of the cathode I I of cathode-ray tube It is'caused to vary in accordance with the signal to be reproduced by means of a conductive connection to the anode l3 of video-frequency amplifier 8, the potential of the cathode I! thus varying in accordance with variations of potential of the anode I3. Suitable operating potentials are provided-for the several tubes of the receiver in a conventional manner.

At the temperatures at which heaters in cathode-ray tubes operate the insulation between the heater and the cathode is usually had. It 15,.

therefore, desirable to prevent the heater assuming a potential greatly diiferent from that of the cathode and for this purpose a leak resistance 34 may be connected between the heater and the cathode. As the capacity to earth of the secondary winding of the heater transformer is generally large the value of the resistance must be high in relation to the anode load resistance of the pentode valve in order to make the impedance of the shunt path to earth small.

In considering the operation of the portion of the system just described, it will be seen that, when operating potentials are applied to the receiver, a high unidirectional voltage is applied to the anode i3 of video-frequency amplifier 8. Due to the fact that the cath'ode heater of tube 8 takes an appreciable time to heat the cathode It to an emitting temperature, there is an appreciable period during which little or substantially no space current flows through the load circuit i4, i5 of tube 8 and its anode i3 is at maximum positive potential. electrode 20 of tube 18 is held, during this period, at a potential more negative than that oi. the anode l3 of tube 8 and cathode I! of cathode-ray tube I8, the tube develops no cathode ray and, therefore, deleterious effects on its fluorescent screen during this interval are prevented. As the current in the load circuit of tube 8 increases, the potential of its anode and, therefore,

. the potential ofcathode II, decreases until an operative condition for cathode-ray tube I8 is established. By the time this operative condition is established, the thermionic tubes of the scanning system of the receiver have also warmed up and the scanning system has thus reached an operative condition and a complete raster is scanned on the fluorescent scre'en-of 'tube I8 for normal signal reproduction.

Thus, the load inductor i4 and resistor ii of amplifier 8 comprise means for maintaining the anode potential of amplifier tube 8 above normal during its warm-up period to maintain'the cathode ray of tube l8 extinguished during the initial warm-up period of the thermionic tubes of the scanning system 34, 35. The grid 20 of the'cathode-ray tube l8, the biasing circuit therefor including the resistor 22, and the conductive connectlon of the cathode i1 and anode I! of the respective tubes 18 and 8 comprise means responsive -to the space current, or the anode potential, of the amplifier tube 8 for biasing the cathoderay tube l8 to cutofl for values of the space:

current below a predetermined minimum value or for values of the anode potential above normal and for reducing the bias of the cathode-ray tube to a normal operating value for values 01' the space current in excess of this predeterminedminimum value or ior substantially normal values of anode potential. The space current of tube 8 is less than the aforesaid predetermined minimum value during its warm-up period, and correspondingly its anode potential is above normal during this period and, as before stated,

Since the control Stated in another way, the biasing circuit for thegrid 20 of tube l8, which circuit includes the resistor 22, comprises means for maintaining the I potential of the control grid 20 substantially conreproduction of the signal by tube l8, and the load circuit of tube 8 including the inductor l4 and resistor l5 comprise means dependent upon a substantial-output from the amplifier 8 for biasing the cathode I! to an operative potential. The amplifier tube 18 has substantially no output during its warm-up period and, as before stated, the warm-up period of tube 8 is so proportioned that it is not less than the warmup period of the tubes of the scanning system Sincethe cathode circuit -of cathode-ray tube I 8 includes only the anode load resistor i5 of video-frequency amplifier 8, the other resistance in the circuit being negligible, degeneration of tube I8 is reduced to an unobjectio'nable value. I While there has been described what is at pres ent considered to be the preferred embodiment of this invention, it will be obvious to those age said cathode-ray tube, and means for maintaining said cathode ray extinguished during said initial warm-up period comprising means for maintaining the'anode potential of said firstnamed vacuum tube above normal during its warm-up period, and means responsive to values .of said anode potential above normal formaintaining said cathode ray extinguished, the warmup period of said first-named tube being proper tioned to be not less than the warm-up period of said tube of said scanning means.

3. A television receiver comprising, a signaltranslating stage including a thermionic vacuum tube having an anode, a cathode-ray tube for refrom the invention, and it is, therefore, aimed in the appended claims to cover all such changes v and modifications as fall within the true spirit and scope of the invention.

What is claimed is: 1. A television receiver comprising, a signaltranslating stage including a thermionic vacuum tube, a cathode-ray tube for reproducing a desired received television signal, said tube includduced insuflicient .scanning fields to deflect said cathode ray, therebytending to damage said cathode-ray tube, and means for maintaining said cathode ray extinguished during said initial warm-up period comprising means responsive to the space current of said first-named tube for biasing said cathode-ray tube to cutoff for values of said space current below a predetermined minimum value and for reducingthe bias of said cathode-ray tube to a normal operating value for values of said space current in excess of said predetermined minimum value, said space cur-,

rent of said first-named tube being less than said predetermined minimum value during its warm- 7 up period and said first-named tube having a warm-up period not less than the warm-up period of said scanning tube means.

2. A television receiver comprising, a signaltranslating stage including a thermionic va'cuum producing a desired received television signal,

said tube including a thermionic cathode, means for producing scanning fields to deflect the cathe ode'ray of said cathode-ray tube in accordance with a predetermined scanning pattern, said lastnamed means comprisingthermionic tube means having an initial warm-up period substantially longer than that of said cathode-ray tube oath ode during which there are produced insufliclent scanning fields to deflect said cathode ray, there 4 vby tending to damage said cathode-ray tube, and

means for maintaining said cathode ray extinguished during said initial warm-up period comprising means for maintaining the anode potential of said first-named vacuum tube above normal during its warm-up period, and means re-' sponsive to values of said anode potential above normal for maintainingsaid cathode ray extinguished, the warm-up period of said firstnamed tube being proportioned to be not less than the warm-up period of said tube oi. said scanning means. 1

4. A television receiver comprising, a videofrequency amplifier includinga thermionic vac uum tube having an anode and a conductive load circuit connected thereto, a cathode-ray tube including a control grid and a thermionic cathode for reproducing a desired received television signal, means for producing scanning fields to defleet the cathode ray of said cathode-ray tube in accordance with a predetermined scanning pattern, thermionic tube means having an initial warmup period during which there are, produced insuftube, and means for maintaining said cathode.

ray extinguished during said initial warm-up period comprising means for maintaining the potential of said control grid substantially constant during the reproduction of said signal, means comprising a conductive connection between said load circuit and said cathode for varying the potential of said cathode in accordance with variations of potential of said anode, thereby to effect reproduction of said signal by saidcathoderay tube, and means for maintainingthe potentube having an anode, a cathode-ray tube for reproducing a desired received.-televisionsignal, said tube including a thermionic cathode, means for producing scanning fields to deflect the oathode ray of said cathode-ray tube inv accordance with a predetermined scanningpattern, said lastnamed means comprising thermionic tube means having an initial warm-up period during which. there are produced insuflicient scanning fields to deflect said cathode ray, thereby tending todam- .tial of said anode sufiiciently higher than normal during its warm-up period that said cathode ray is extinguished, the warm-up period of said amplifier tube being proportioned to be not less than the warm-up period of said tube of said scanning means.

5. A television receiver comprising, a video-frequency amplifier including a thermionic vacuum tube, a cathode-ray tube including a control grid and a cathode for reproducing a desired received signal, means for producing scanning fields to deflect the cathode ray of said cathode-ray tube in accordance with a predetermined scanning pattern, said lastmamcd means comprising said last-named means comprising thermionic tube means having an initial Warmup period during which there are produced insufflcient scanning fields to deflect said cathode ray, thereby tending to damage said cathode-ray tube, and means for maintaining said cathode ray extinguished during said initial warm-up period comprising means for maintaining the potential of said control grid substantially constant during the-reproduction of said signal, means for varying the potential of said cathode in accordance with the signal output of said amplifier,

thereby to eflect reproduction of said signal by said tube, and means dependent on a substantial output from said video-frequency amplifier for biasing said cathode to an operative potential, said amplifier tube having substantially no output during its warm-up period and said warm-up period being so proportioned that it is not less than the warm-up period of the tube of said scanning means.

6. A television receiver comprising, a signal repeater including a conductive load circuit, a cathode-ray tube including a control grid, a cathode and a cathode heater for reproducing a desired received television signal, means for maintaining the potential of said control grid substantially constant during signal reproduction, means comprising a conductive connection between said load circuit and said cathode for varying the potential of said cathode in accordance with said desired received ignal, thereby to effect reproduction of said s gnal by said cathode-ray tube, and means including a resistor connected between said cathode and said heater, the resistance value of said resistor being large relative to the resistance of said load circuit.

MAURICE KENYON TAYLOR. HUBERT WOOD. 

